The present invention relates to a four-wheel drive system of a part-time type having a four-wheel drive mode and a two-wheel drive mode, and more specifically to an automatic control system for a part-time four-wheel drive system, which is arranged to change automatically from the two-wheel mode to the four-wheel mode when the braking force of the vehicle is increased, in order to improve the braking performance.
A four-wheel drive vehicle is superior in ability of hill climbing and ability of rough road driving. If, however, all four wheels are always driven at the same speed, a four-wheel drive vehicle cannot be turned smoothly. During a turn with large steering angles of steerable wheels, the front wheel of an inner or outer side must travel along a circle having a large radius and the rear wheel of the same side must travel along a small circle. Because of this difference in turning radius, there arises a large difference between a rotation speed (an average rotation speed, to be exact) of the front wheels and a rotation speed (an average rotation speed) of the rear wheels. As a result, the steering becomes heavy, the tendency to understeer is increased, and the vehicle cannot be turned without abnormal tire friction (called tight corner brake) which tends to brake the vehicle and cause an engine stall.
To overcome these handling and tire friction problems, some four-wheel drive vehicles are provided with a changeover means capable of changing the state of a four-wheel drive system between a two-wheel drive mode and a four-wheel drive mode. This type of four-wheel drive is called a part-time type. Some examples of the part-time four-wheel drive vehicle are shown in Japanese Patent provisional publications No. 55-83617 and No. 56-75220.
A part-time four-wheel drive vehicle can be turned smoothly in the two-wheel drive mode. Furthermore, a part-time four-wheel drive vehicle has an excellent braking performance when the four-wheel drive mode is used. The front wheels and the rear wheels are forced to rotate integrally by the drive system in the four-wheel drive mode, so that, during hard braking, the front and rear wheels are forced to stop simultaneously. Because of this nature, a part-time four-wheel drive vehicle in the four-wheel drive mode can provide an excellent braking performance. However, the excellent braking performance of a four-wheel drive vehicle can be obtained only when the four-wheel drive mode is used. It is very difficult or practically impossible for a drive to change manually the drive system from the two-wheel drive mode to the four-wheel drive mode during hard braking.